Dominant Aeroacoustic Source Contribution in a Hot Transonic Jet

Abstract

The near-fleld region of a Mach 0.6 axisymmetric jet, Tr=1.72 and exit nozzle diameter of 50.8mm, is examined experimentally with the aim of characterizing the dominant aeroacoustic sources. PIV measurements are acquired at downstream locations subsequent to the collapse of the potential core region, where acoustic sources have shown to exhibit the strongest contribution to the far-fleld noise. The ∞uctuating pressure fleld is simultaneously sampled at 2D downstream of the jet exit, within the hydrodynamic region, by an azimuthal array of 15 transducers. Application of a low-dimensional technique, combines the discretely sampled velocity fleld with the temporally resolved pressure measurements to reconstruct an estimate of the ’large-scale’ velocity fleld. Six far-fleld microphones positioned on an arc at 75D from the jet centerline serve as a measure of direct comparison, as correlations with the low-dimensional, estimated velocity fleld illustrate the spatial and temporal extent of noise producing events. This study comprises two temperature ratios, Tr=1 and Tr=1.72 in an efiort to validate this approach, isolating the efiect of temperature, as noise sources are known to be altered at elevated temperatures. he problem of jet aircraft noise continues to be the focus of many researchers. The source of this noise can be linked to the turbulent mixing of the exhausted jet plume with the ambient air; thus creating pressure ∞uctuations in the near-fleld and acoustic-far fleld regions of the jet ∞ow. The role of turbulent shear ∞ows in generating vortical structures, and the noise radiated by this unsteady ∞uid motion, has recently gained the renewed interest of many researchers. Numerous investigations have been conducted aimed at characterizing the near-fleld region of the turbulent axisymmetric jet, with the challenging objective of accurately relating the acoustic source to its response in the far-fleld. The ’source’ being deemed as the generation, subsequent interaction, and evolution of the coherent turbulence structure, as cited by Ffowcs Williams. 3 One of the more signiflcant contributions, though not at all recent, being the establishment of Lighthill’s 7 aerodynamic noise theory. In the application of Lighthill’s acoustic analogy, an acoustic approximation to momentum transport, detailed knowledge of the ∞uctuating velocity fleld in those ∞ow regions of strong acoustic energy is required. However, in-∞ow measurements acquired using such tools as hot-wire anemometry, proved intrusive; ultimately corrupting the ’true’ far-fleld acoustic signature, making quantitative analysis di‐cult. In an attempt to circumvent these types of measurements,researchers such as Ko & Davies 6 directed their efiorts towards understanding the relationship that coupled the ’source’ with external ∞uctuating quantities, like near-fleld pressure. These measurement however, are acquired in the region of transition from non-linear acoustic sources of sound to linear wave propagation observed in the acoustic far-fleld, making any inferences about the source characteristics obscure. Experimental examination of this aeroacoustic efiect at high temperatures also adds an additional challenge, as noise sources have been shown to be altered at elevated temperatures, at which realistic engine cycles occur. Far-fleld noise levels